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  <div class="titlepage"><div><div><h1 style="clear: both">Define a custom code domain</h1></div></div></div>
  <p>Firstly let&rsquo;s alias the experimental Outcome namespace into something
less tedious to type, declare our custom status code type, and get
started on defining the custom status code domain implementation.</p>

<div class="code-snippet"><div class="highlight"><pre class="chroma"><code class="language-c++" data-lang="c++"><span class="k">namespace</span> <span class="n">outcome_e</span> <span class="o">=</span> <span class="n">BOOST_OUTCOME_V2_NAMESPACE</span><span class="o">::</span><span class="n">experimental</span><span class="p">;</span>

<span class="c1">// To define a `file_io_error` which participates in the P1028 world
</span><span class="c1">// of `std::error`, we must first declare, then define, a custom code
</span><span class="c1">// domain which extends `posix_code` (the std error coding for POSIX
</span><span class="c1">// failures). The following is fairly standard boilerplate for defining
</span><span class="c1">// a custom code domain. It is analogous to defining a custom `std::error_category`.
</span><span class="c1"></span>
<span class="k">class</span><span class="err"> </span><span class="nc">_file_io_error_domain</span><span class="p">;</span>
<span class="c1">// We define `file_io_error` to be the status code whose domain is `_file_io_error_domain`.
</span><span class="c1"></span><span class="k">using</span> <span class="n">file_io_error</span> <span class="o">=</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">status_code</span><span class="o">&lt;</span><span class="n">_file_io_error_domain</span><span class="o">&gt;</span><span class="p">;</span>

<span class="c1">// Now we define `_file_io_error_domain`.
</span><span class="c1"></span><span class="k">class</span><span class="err"> </span><span class="nc">_file_io_error_domain</span> <span class="o">:</span> <span class="k">public</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">posix_code</span><span class="o">::</span><span class="n">domain_type</span>
<span class="p">{</span>
  <span class="k">using</span> <span class="n">_base</span> <span class="o">=</span> <span class="k">typename</span> <span class="n">outcome_e</span><span class="o">::</span><span class="n">posix_code</span><span class="o">::</span><span class="n">domain_type</span><span class="p">;</span>
</code></pre></div><a href="https://github.com/boostorg/outcome/tree/master/doc/src/snippets/experimental_status_code.cpp#L66" class="code-snippet-url" target="_blank">View this code on Github</a></div>


<p>Note that we inherit from <code>outcome_e::posix_code::domain_type</code>, not
from <code>outcome_e::status_code_domain</code>. We thus reuse most of the
implementation of <code>outcome_e::posix_code::domain_type</code> rather than
implementing required functionality. If you would like to see a
fuller treatment of defining a custom status code domain from
scratch, see <a href="https://github.com/ned14/status-code/blob/master/doc/custom_domain_worked_example.md">this worked example here</a>.</p>

<p><a href="https://ned14.github.io/status-code/doc_status_code_domain.html#standardese-system_error2__status_code_domain">You may find looking at the API reference documentation for <code>status_code_domain</code>
useful in the next few pages </a>.</p>


        </div><p><small>Last revised: January 26, 2019 at 23:38:56 UTC</small></p>
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